Water uptake depth of European beech, Douglas fir and Norway spruce species-specific patterns, seasonal dynamics and mixture effects

Trees are major regulators of the forest water balance. To stay vital, water loss via transpiration has to be compensated by root water uptake. However, it is often not clear where exactly the trees get their water from. This has become particularly relevant in central Europe, as forests are facing more frequent and intense droughts with climate change. It is known that water uptake strategies are species-specific, influenced by environmental conditions and, potentially, neighboring species. Yet, knowledge about species-specific patterns of water uptake depth and how it is affected by tree-species mixture is scarce. Stable water isotopes present a valuable tool to elucidate these belowground processes.For our study, we selected mixtures of European beech (Fagus sylvatica), the dominant broadleaved tree species in central Europe, with native, but drought-prone Norway spruce (Picea abies), and non-native, but supposedly more drought-resistant Douglas fir (Pseudotsuga menziesii), as well as the respective pure stands. We aimed to uncover the effect of (1) species identity, (2) species mixture and (3) environmental conditions on water uptake depth. To achieve this, we conducted two sampling campaigns in climatically contrasting years: a natural abundance campaign in relatively wet 2021, covering 20 plots on 4 sites, and a tracer experiment focused on European beech and Douglas fir on a subset of plots, including weekly sampling of 12 trees throughout the drought summer 2022.We found species-specific patterns of water uptake depth, where Norway spruce tended to use the greatest share of shallow water, followed by European beech and Douglas fir. Within species, the data indicated differences in water uptake depth between pure and mixed stands, however, we did not detect a spatial differentiation between co-occurring species. Dry conditions tended to shift water uptake to deeper layers, with beech responding stronger than Douglas fir.Our results corroborate that species-specific traits have to be considered when assessing forest water pathways, especially in mixed forests and under drought. Considering central Europe, our data supports the assumption that Douglas fir may be more drought resistant than Norway spruce by tapping deeper water sources. In mixture, both European beech and Douglas fir seem to exploit similar soil depths, while none of the species is limited to the drought-prone topsoil.